1. Field of the Invention
The invention relates to a device and a process for signal analysis. The signals can be of general form and can be present especially as one-dimensional, particularly time signals. Signals of this type in the area of engineering are derived for example from noise signals, as are caused by running machinery. Another example from the area of engineering is signals which represent a regular machine state, or which are characteristics of a brief or continuing defect. These defects can relate to the machinery itself, or a product produced by this machinery.
2. Description of Related Art
The numerous and varied signal analysis processes of type to which this invention is directed are known. If the ratio between a signal portion of interest and a noise portion is large enough, analysis is usually less difficult. But, it is of special interest and considerable difficulty to extract those signal portions which are strongly adulterated by noise. Likewise, it is of special interest to identify individual signals or those which occur only briefly or sporadically and possibly stochastically when a host of other signals is present at the same time.
For example, it is known that it is extremely difficult to identify bearing noise in a machine when the signal/noise ratio becomes less than a value of roughly 0.25 (compare British Journ. of NDT, Vol. 35 No. 10, p. 574 ft). The use of conventional Fourier analyses has almost no effect in this case.
Furthermore, a device for detecting or analyzing machinery damage of a type to which this invention is also directed, especially for detection of errors on roller bearings, is known from U.S. Pat. No. 3,842,663. This patent explains a standard problem which occurs when machinery or bearing damage is detected. This is based on the fact that the noise periodically emitted by bearing defects or other defective hardware parts is much smaller than regular noise which is produced by this machinery and may be present as solid-borne noise.
The approach presented in U.S. Pat. No. 3,842,663 is based on acquiring machinery noise, such as for example transmission or bearing noise, by means of a suitable detector and converting it into an electrical signal. In this case, the damaged bearing or machine parts produce pulse-like noise portions which preferably excite the natural resonances of this detector. Its electrical output signal is processed by a pre-filter and a demodulator. The resulting signal then corresponds essentially to a pulse series. In this pulse series, an individual pulse represents an individual noise caused by sudden motion components.
In periodically repeated pulses of this type, it is possible to determine an especially pronounced or emerging signal frequency with an output-connected spectral analyzer. This frequency, at a known base frequency of the machinery or certain rpm and characteristics of a bearing, allows a conclusion regarding a defect within the machine or bearing.
U.S. Pat. No. 5,679,900 describes how the aforementioned device can be used by using a switchable filter in a paper-making machine. This document also considers that the machinery defects can be characterized by pulse-like vibration portions compared to regular machinery noise, and that they can be emphasized by suitable vibration sensors and by suitable signal processing against the base noise of the machinery and mechanical assemblies.
But, the described devices and processes are subject to the disadvantage that efficient, and thus expensive, spectral analyzers must be provided for reliable representation of the results obtained or additional complex filter units must be connected on the input side.
In addition, the pulse like vibration portions tend to be characterized by significant amplitude, but of rather little energy content. This is because of their short duration. In terms of energy, such pulse-like vibration portions are likely to be xe2x80x9cburiedxe2x80x9d in background noise and thus difficult to impossible to find.
The primary object of the invention is to improve the quality of analysis results and to significantly reduce the required effort for detecting machine damage, especially bearing damage, by cost-favorable electronic circuitry or data processing.
Another object of the invention is to make available a device and process for improved signal analysis which is especially suitable for detection of an early stage of incipient bearing or machinery damage.
It is also the object of the invention to devise a device and process by which improved, more effective analysis or detection is possible for recognition of those signal portions which are characterized by only a brief presence in time, i.e., transient signal effects. The process in accordance with the invention can, moreover, be used, for example, for detection of very noisy signals.
The invention is based on the finding that a sampled time signal usually present in digitized form can be interpreted as an ordered set of individual events with an inner correlation which is initially unknown. The invention is furthermore based on the finding that the results of conventional, known correlation analyses can be supplemented and improved by abandoning the practice of combining with one another of sections of these signals which are shifted relative to one another in time. Rather, in accordance with the invention, each individual event (according to the definition above) of a signal to be examined is combined at least once with one or more other individual events, which are separated from each other by a time difference. Then, the results of the individual combinations are sent for classification. In particular, it is of interest according to the invention to provide for classification purposes one such combination in the manner of a distribution which has been formed by summation. The x-axis of this distribution is preferably arranged by time classes. The individual time classes correspond to the time differences so that the results of the individual combinations can be fitted into the distribution using the underlying time differences. It is advantageous for combination of the events to perform multiplications each time. However, for this purpose, other binary logic operations can be used, for example, summation or determining a maximum.
According to one preferred approach of the invention, it is provided that each event be combined with all its predecessors or with those predecessors which are located in its immediate vicinity. According to another approach of the invention, it is provided that each event be combined with any other and the results of the combination likewise be sent for classification.
According to still another implementation of the invention, it is provided that events of predefined xe2x80x9cqualityxe2x80x9d (i.e. having predefined characteristics) be preselected for constituting a xe2x80x9cfilteredxe2x80x9d signal, consisting of such specific events which then are to be combined with any other, and the results of the combination be sent for classification in a comparable way.
In accordance with the invention, the classification results obtained by a distribution analysis can be evaluated either directly or can be sent for additional analysis. Here, it is especially useful to normalize those extreme values of a resulting frequency distribution by a mapping rule which can be interpreted to relate to an integral multiple of a time difference between two predetermined extreme values. One such extreme value, therefore, belongs to a time class of the distribution which is characterized by a smaller (difference) time value than the time classes of the integral multiple of the first extreme value.
In the direct evaluation of the resulting classification result (i.e. classification function), it can be enough to check the function values of individual time classes with reference to a predefined threshold value. Another direct evaluation involves, for example, checking the observed extreme values, especially maximum values, of a resulting distribution. According to the present invention, the result of these comparisons then yields the desired conclusion, whether repetitively occurring signal portions of a xe2x80x9cnoisyxe2x80x9d signal can be considered detected or not.
In an additional type of evaluation according to the invention, a resulting distribution can be transformed, for example, by means of a Fourier transform, from the time domain into the frequency domain. In this way, it is possible to acquire an idea of whether and which values of a resulting frequency distribution are present as time-multiples, roughly in the manner of a so-called xe2x80x9ccepstrumxe2x80x9d analysis.
In special embodiments, the inventive device is characterized by the following partially optional features:
There is a sensor for acquiring machinery noise and for conversion into an electrical signal, and preferably, an analog or digital rectifier for forming the absolute value of the electrical signal.
The device determines, from a signal caused by the machinery noise, special results which are defined in terms of time and intensity.
The device has one, preferably two or more, peak value detectors with different recovery times or decay time constants.
The device has one or more time analyzers for analysis of the output signals of the peak value detectors. In this case, the time analyzers can determine the respective instants of intermediate maximum or minimum values of the electrical signal as time events. The respective time points together with the pertinent signal amplitudes can be stored in a buffer.
The device can form product values from stipulated signal amplitudes of selected time points and can perform frequency analysis.
The device can execute the indicated frequency analysis by time classes which can be essentially freely selected according to predetermined assessment criteria.
The device can undertake frequency analysis using a weighted distribution.
In the execution of frequency analysis for evaluation of a machine or bearing state, the device uses only those events which have a sufficient or significant difference from a noise level.
The device is equipped with a unit for acquisition and proportional subtraction of those functional values which correspond to a multiple of a base time value.
The device is equipped with an evaluation unit by which a frequency measure which belongs to a base time value is evaluated in that whether, what type, and to what extent there is machinery or bearing damage.
The device is equipped with a registration unit with which predefined time values and base time values can be stored over a longer time interval so that at a later time measured in weeks or months, analysis can be performed and a display of the degree of damage for a bearing or for a machine component is possible over greater time intervals.
The device has means with which it is possible to use two or more events of a signal which follow one another directly in time to prepare and store a product or a product sum for the intensities of these events. The products or product sums can be supplied for frequency analysis and subsequent evaluation.
The device has means with which it is possible that relatively large time differences for events can be assumed and processed. The time differences are at least as great as corresponds to a maximum expected repetition time for an event which can be assigned to a special type of damage.
The device can have threshold detectors with decay time constants which agree in terms of order of magnitude with the repetition times or period durations of the effects of the machinery damage or defects to be examined.
The device has means with which a resulting event value is produced by a logic operation from two event intensities at a time (specially defined intensities of the electrical signal) and the logic operation is effected by product formation, by summing, by determining a maximum value or by another relation.
The device can have a means for digital or analog preprocessing of the signal, for rectification or for absolute-value generation of a signal.
The device can have a means for identifying the intermediate extreme values (maximum or minimum values) of the signal. Using such means, an event can be classified according to the instant of its occurrence and its intensity.
For these reasons, the device advantageously has one or more peak value detectors, each of which is characterized by a defined recovery time or decay time constant.
The device has a means for forming and evaluating several frequency distributions in a multiplicity of damage duration categories.
The device can have a selection stage which provides for an event defined in time, triggering or influencing only that peak value detector which is characterized by the time constant which is the largest at the time.
The above described or additional features of the invention can be formulated alternatively or optionally as follows:
The device for detecting or analyzing machinery damage is especially suited for detecting repeated, pulse-like portions of a time-variant signal and especially for recognizing machinery noise which is caused by roller bearing damage, and the device has the following components:
a sensor, especially in the form of an acceleration sensor, for acquiring and converting the time-variant signal into an electrical AC signal,
especially at least one filter stage and one signal conversion stage for producing positive definite signal portions, i.e., no negative values result.
at least one peak value detector stage with preset discharge time constant for detecting the time occurrence of individual relative extreme values (maximum values or minimum values) of the AC signal,
a storage unit which stores the times and the intensity (amplitude) of the recognized individual relative extreme values over a predetermined time window (observation interval),
a computer which can multiplicatively combine the recognized individual relative extreme values, i.e. their assigned intensities, so that corresponding product values are produced,
a classification unit for processing of product values which can compute and store at least one distribution (histogram),
furthermore, a pattern recognition unit for analysis of at least one distribution according to preset criteria, especially according to predefined absolute frequency values.
The device can have one or more additional reference signal generators for generating sawtooth reference signals with a steep rise and comparatively slow fall; furthermore, one or more trigger stages for repetitive triggering of the reference signal generators.
The device can have a signal analysis stage in which sampled, time-discrete signals are used to trigger one or more of the trigger stages. These signals can then be compared to the sawtooth reference signals.
The device is able to store different special samples according to their respective instants of occurrence and pertinent signal intensities (amplitude). Furthermore, it has a computer for forming arithmetic products, a classification unit for forming one or more histograms and a pattern recognition unit for classification of histograms according to predetermined criteria.
The device has an analysis unit by which a large number of events can be determined and a large number of logic operations can be sent for histogram analysis until an output by the analysis unit or an evaluation unit according to stipulated statistical criteria can be regarded as being steady-state or statistically relevant.
The device is equipped with one or more comparison stages for checking that stipulated boundary values havexe2x80x94alternativelyxe2x80x94been exceeded, have not been reached, or are identical; and the device can have, in particular, a signaling stage. The signaling stage is used for signaling if preset boundary values, filed in the form of histogram values, have been exceeded.
The device can have a digital computer with which time events are determined and with which the data of these events are further processed.
The invention is also based on the finding that the time length of the signals which occur in pulses on the machinery or bearings and which are caused, for example, due to defects or deformation of bearing parts is generally rather short. For this reason, their determination by means of a Fourier transform is difficult due to the other noise components present on a bearing or a machine.
Therefore, according to the invention, it is more advantageous to identify the fraction of pulse-like signal portions per individual event. With these individually determined events, it is then easier to identify the parameters of their occurrence in time. With the data obtained in this way, underlying causes can also be determined in an early stage of damage of machinery or bearings, especially in roller bearings.
However, it is generally not enough to check an individual frequency of the noise spectrum. Rather, e.g. for roller bearings one has to check at least four (4) basic separate frequencies which could indicate damage. These frequencies are referenced, for example, to the dimensions of the inner ring, the outer ring, the rollers and the cage of a roller bearing. Since these frequencies can occur in any combinations, the teaching of U.S. Pat. No. 3,824,663 is only of limited benefit.
With the object of identifying the individual pulse-like noise portions of a faulty machine or bearing, in accordance with the invention, it is provided that these noise portions be analyzed by means of detection for intermediate maximum and minimum values of the acquired and filtered noise signal without the need for a more complex frequency analysis unit, such as a FFT analyzer. One approach according to the invention is to provide one or more peak value detectors which study a prefiltered signal, after its rectification or absolute value generation, for conspicuous time points and the pertinent signal intensities.
Here, it is very advantageous for the individual peak value detectors to have different decay times. In this way, pulse-like noise events, hereinafter simply called events, can be better studied and identified both in different time-scale domains and also in different intensity ranges. According to the invention, it is especially advantageous to study, not individual events and their assigned formation times, but the combination of at least two events which are adjacent in time or apart in time. The combination of at least two events is preferably undertaken such that a multiplicative combination of the two intensity values of the respective events is performed. The statistics to be determined for the product values formed in this way and the pertinent time differences allow conclusions to be drawn about the composition of the repetition times involved, and thus, the nature of underlying machine damage or roller bearing damage. Under certain circumstances, it is enough to study a correspondingly formed frequency distribution or the statistics of the event products over assigned difference times to ascertain whether predefined time domains which can be assigned to one of the aforementioned roller bearing damage frequencies exceed a stipulated threshold or not.
In the invention, it is assumed that it is generally enough to analyze a time interval which comprises only a few to a few hundred seconds. The signals belonging to this time interval can be digitized and stored in a digital memory. Furthermore, in accordance with the invention, it is assumed that a plurality of the described means, functions and processes can be executed not only with analog electronic means, but alternatively or additionally with digital ones, i.e. computer-based. The latter approach then uses the corresponding software for analogous implementation of the means, functions and relationships provided in accordance with the invention.
These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention.